From 7459c021874246c196f397686e100702f059b9e7 Mon Sep 17 00:00:00 2001 From: Julian Sun Date: Tue, 15 Sep 2026 12:49:12 +0800 Subject: [PATCH] fs: avoid repeated scans in evict_inodes() We observed hung tasks when users attempted to unmount a filesystem after its disk had been removed while still in use. During device removal, fs_bdev_mark_dead() calls evict_inodes() while holding s_umount. Each time evict_inodes() drops s_inode_list_lock to reschedule, it restarts the walk from the head of s_inodes. With many referenced inodes at the head of the list, these restarts repeatedly scan the same inodes without reclaiming them. This can keep s_umount held for a long time, blocking concurrent umount attempts and triggering hung-task reports. Keep the current inode, already marked I_FREEING, out of the disposal batch until s_inode_list_lock is reacquired. Resume the walk from this inode and dispose of it in a later batch or at the end of the walk. The zero-refcount and state checks under i_lock allow this walker to claim the inode by setting I_FREEING and removing it from the LRU. Other reclaimers skip the inode, leaving this walker responsible for eviction. Only evict() removes it from s_inodes, so keeping it out of the disposal batch ensures that it remains on the list while the lock is dropped. After reacquiring the lock, reading its current next pointer accounts for concurrent removal of following inodes. The existing inode lifetime rules prohibit acquiring a reference to an inode marked I_FREEING or I_WILL_FREE. __iget() requires its caller to hold i_lock and establish that taking a reference is valid. Inode lookup and igrab() check these flags under i_lock when acquiring a reference from zero. ihold() requires an existing reference, which would keep i_count nonzero and prevent this walker from claiming the inode. These rules already allow iput_final() and the inode shrinker to release i_lock after setting I_FREEING and before eviction completes. A temporary __iget() reference would also keep the inode on the list, but its release must preserve last-reference handling. Another user can acquire a reference, update lazy timestamps and drop its reference while the pin is held. If the pin becomes the last reference, dropping it with atomic_dec_and_test() and evicting directly bypasses iput()'s lazytime handling and can lose those timestamp updates. Releasing the pin with iput() preserves that handling, but does not guarantee eviction. fs_bdev_mark_dead() runs with SB_ACTIVE set, so iput() may retain the inode in cache, whereas evict_inodes() must evict eligible zero-reference inodes. The inode may also have been freed when iput() returns, so the walker cannot then use it to force eviction. Using I_FREEING preserves the existing eviction behavior without introducing an additional last-reference transition. The xfstests auto group passed on ext4 and XFS with known unrelated failures excluded. No new issues were observed, and the previously reproducible hung task no longer occurs with this patch. Fixes: ac05fbb40062 ("inode: don't softlockup when evicting inodes") Signed-off-by: Julian Sun Link: https://patch.msgid.link/20260915044912.3183440-1-sunjunchao@bytedance.com Reviewed-by: Jan Kara Signed-off-by: Christian Brauner (Amutable) --- fs/inode.c | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-) diff --git a/fs/inode.c b/fs/inode.c index ba7da39be4a3..a9d37be390a1 100644 --- a/fs/inode.c +++ b/fs/inode.c @@ -880,7 +880,6 @@ void evict_inodes(struct super_block *sb) struct inode *inode; LIST_HEAD(dispose); -again: spin_lock(&sb->s_inode_list_lock); list_for_each_entry(inode, &sb->s_inodes, i_sb_list) { if (icount_read_once(inode)) @@ -899,19 +898,19 @@ void evict_inodes(struct super_block *sb) inode_state_set(inode, I_FREEING); inode_lru_list_del(inode); spin_unlock(&inode->i_lock); - list_add(&inode->i_lru, &dispose); /* - * We can have a ton of inodes to evict at unmount time given - * enough memory, check to see if we need to go to sleep for a - * bit so we don't livelock. + * Keep this inode out of dispose so it stays on s_inodes while + * the list lock is dropped. I_FREEING prevents new references + * and leaves eviction to us, so we can resume the walk from it. */ if (need_resched()) { spin_unlock(&sb->s_inode_list_lock); cond_resched(); dispose_list(&dispose); - goto again; + spin_lock(&sb->s_inode_list_lock); } + list_add(&inode->i_lru, &dispose); } spin_unlock(&sb->s_inode_list_lock);